1. Field of the Invention
The present invention relates to magnetic toners including magnetic powder for developing electrostatically charged images in electrophotographic methods, electrostatatic-printing recording methods, and the like.
2. Prior Art
In general, electrophotographic methods comprise the steps of: forming an electric latent image on a sensitizing material; developing the latent image with toners to form a toner image; optionally transferring the toner image to a decalcomania material such as paper; and fixing the toner image by means of heating, pressurization, and the like to obtain a copy. Classes of developers for use in such electrophotographic methods include two-component developers consisting of a toner and a carrier, and single-component developers consisting of only a toner which also functions as a carrier.
As the single-component developer, so-called magnetic toners can be used. The magnetic toners include magnetic powder in an amount of approximately 10% to 70%. Generally, magnetic toners are roughly divided into conductive magnetic toners and insulating magnetic toners. The insulating magnetic toners have been used not only in single-component contact or non-contact developing systems, but also in two-component developing systems with appropriate carriers.
In such a single-component developing system, it is extremely important that the magnetic toners retain triboelectrification (triboelectrification: the production of electrostatic charges by friction), since the single-component developer includes no carriers functioning to accelerate triboelectrification of the magnetic toners. Namely, a "triboelectrification property" which means that triboelectrification of magnetic toners speedily reaches a saturated value by causing the magnetic toner particles to come into light contact with one another or with a doctor blade or the like, largely affects durability of the magnetic toners and developing characteristics such as image density, smudging, image quality, and the like.
In the two-component developing system mentioned above, a suitable triboelectrification is necessary in order to obtain stable developing characteristics at low toner-density as well as at a high toner-density, since almost all developing machines used in the two-component developing system are not sophisticated enough to control toner-density.
In addition, since a magnetic toner particle is a mixture of magnetic powder, a binder resin, an electrostatic charge control agent, and the like and such materials tend to exist nonuniformly on the surface of the magnetic toner particles, each magnetic toner particle does not always have uniform triboelectrification properties. Therefore, in order to obtain magnetic toner particles having uniform triboelectrification, it has been proposed that developing characteristics can be improved by improving uniformity of the size of the magnetic toner particles by classifying such as to remove coarse particles and fine particles; or adhering or fixing various additives which participate in the triboelectrification on the surface of each magnetic toner particle. However, the conventional magnetic toners described above do not have sufficiently uniform triboelectrification properties which are desirable for magnetic toners.